Electrons and holes both move fast in cubic boron arsenide

For the first time, researchers have experimentally discovered that a cubic boron arsenide crystal offers high carrier mobility for both electrons and holes – the two ways in which charge is carried in a semiconducting material – suggesting a major advance for next-generation electronics.

While earlier predictions had theorized that the crystal could exhibit simultaneously high electron and hole mobility, researchers have now experimentally validated the high carrier mobility at room temperature. Researchers from across the US, including the University of Houston (UH), the Massachusetts Institute of Technology, the University of Texas at Austin and Boston College, were involved in the work, which is reported in a paper in Science.

An accompanying Science paper reports the use of transient reflectivity microscopy to characterize the crystal, again demonstrating the high mobility. In certain cases, when a higher-energy laser beam was used, the measured mobility even exceeded previous predictions. This work was done by researchers from UH and from the National Center for Nanoscience and Technology in Beijing, along with several other institutions in China.

Zhifeng Ren, director of the Texas Center for Superconductivity at UH and a corresponding author on both papers, said this work has important implications for a range of electronic and optical applications, similar to the advances that followed the advent of silicon wafers, which are widely used in all kinds of electronics.

Some semiconductor applications require a material with both high thermal conductivity – which measures how effectively a material conducts heat – and high electron and hole mobility. Earlier research had demonstrated that cubic boron arsenide has high thermal conductivity, making the high ambipolar mobility a crucial advance.

“The potential of this material is tremendous,” said Ren, who is also a professor of physics at UH. While work to consistently produce larger crystals with uniform properties is ongoing, this result could have an even bigger impact on the field than the silicon wafer, he said.

That’s because semiconductors require that current be carried via both electrons and holes, but most known materials offer high mobility for only one type of carrier. The overall efficiency of the semiconductor is determined by the lower value.

“If both are high, the device will be more efficient,” Ren said. “That’s what makes this material unique.”

Ren was among a group of researchers who reported in a paper in Science in 2018 that the crystal – grown from boron and arsenic, two relatively common elements – demonstrated far higher thermal conductivity than traditional semiconductors. This work builds on that, using crystals grown in Ren’s lab to demonstrate that theoretical predictions about the substance’s high mobility can be demonstrated experimentally.

Carrier mobility is measured by the unit of cm²V^-1s^-1; the researchers reported mobility in cubic boron arsenide of 1600cm²V^-1s^-1. That portion of the work was led by Gang Chen, professor of power engineering at MIT and co-corresponding author of the paper, using an optical transient grating method to measure both electrical mobility and thermal conductivity.

In the second paper, researchers led by Ren and Jiming Bao of UH and Xinfeng Liu at the National Center for Nanoscience and Technology, reported mobility in a range from about 1500cm²V^-1s^-1 to as high as 3000cm²V^-1s^-1.

Measuring carrier mobility was complicated by the fact the crystal wasn’t large and uniform, meaning traditional measurement methods such as the Hall effect couldn’t accurately determine its properties. According to the researchers, ionized impurities weakened the material’s performance by strongly scattering the charge carriers, although other impurities – described in the paper as ‘neutral impurities’ – had less of an impact.

“The sample was not uniform, but you can see the potential locally,” Ren said. “If you had a crystal free of defects, mobility could be potentially much higher than predicted. We are in continuous research to figure that out.” The measurements were performed using different methods in labs at UH and MIT.

In the second paper, researchers from UH and six Chinese universities and institutions report the use of transient reflectivity microscopy to measure the electron and hole mobility.

Bao, professor of electrical engineering at UH and a principal investigator with the Texas Center for Superconductivity, said the researchers used laser pulses to excite carriers in the sample to monitor their diffusion. In the process, they discovered a key difference between the cubic boron arsenide crystal and most other semiconducting materials.

In silicon, for example, electrons move about four times more quickly than holes. “In this case, the holes move more quickly than electrons,” Bao said. But both electrons and holes exhibited unusually high mobility, improving the material’s overall performance.

Bao attributed the highest measurements, which detected mobility far higher than 1600cm²V^-1s^-1, to ‘hot electrons’, which maintained the heat, or energy, generated by the laser pulse longer than they do in most other materials. The same was true of holes in the material, Bao said.

The structure of the cubic boron arsenide crystal makes it more difficult for the charge carriers to cool, meaning they maintain the heat – and the resulting high mobility – for longer. The researchers reported mobility similar to the predicted levels and those found by Chen’s lab but noted that additional experiments revealed a mobility of more than 3000cm²V^-1s^-1, which they attributed to the hot electrons.

The findings depended in part on measuring a section of the crystal with few or no impurities, Bao said. “The sample was not uniform, and we found the highest mobility at spots with the fewest impurities.”

This story is adapted from material from the University of Houston, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.